Thanks, JMS!

The point I wanted to raise is that early on, the water skeptics were putting forward volcanic ash beds, or a tuff formation as one theory to explain the outcrop. Given what the MI images look like and given the huge sulfur signature (and lack of a silicate signature, apparently), it sounds like the ash hypothesis is looking less likely.

That a fair take, those of you with more geologic knowledge than me?

One thing that seems odd if this is hydrothermal, though, is the huge extent of the formation. It looks like you see the same light-toned stuff cropping out in craters on a regional scale. How do you get hot springs deposits to do that?